subst_xx catalog change

Before 3.50 we provided undocumented/unsupported means to modify the UI by replacing game files using substitution catalog-files, knowing that this way modders who wanted to risk it could at least have a way to tweak the UI before we were able to add official UI modding support.

As of this version this old deprecated feature no longer works for the UI (it's still kept in place for other cases, for which it will still work). Please use the normal UI-modding facility to provide UI modifications.